Gemstone Classification

About the Classification and Types of Gemstones

The first aspect of all manners of classification when it comes to gemstones is the difference between stones that are grouped as precious, and those that are grouped as semi-precious. These distinctions date back to the time of Ancient Greece, and still apply today.

Precious Stones

In the West, the modern consensus is that there are four stones that can be classified as the most precious, which are emeralds, sapphires, rubies, and diamonds. The reasoning behind this categorization is anything but methodical. The main reason was due to the rarity of these four stones in these ancient days gone by. It was also a matter of their qualities; all four of these precious gemstones are considered to have the finest colours (apart from the diamond) when they are found in their purest forms, and all are meant to have incredible translucency, as well as having the most dense matter. A gem's hardness is measured on what is called the Mohs scale, which is basically a measure of the strength and ability of one piece of matter to scratch another. All of the four precious stones mentioned above measure between eight and ten on the Mohs scale; the diamond being the hardest at ten.

Semi Precious Stones

All other gemstones are classed as semi-precious when compared with the four above, and are judged by their hardness, their translucency and their colour. This task of classifying different stones is a job done by 'gemologists' in the area of gemology. And it's quite a scientific undertaking! The first task a gemologist needs to undertake when faced with a stone is to identify its chemical composition. This means, basically, finding out for each element the simplest whole number ratio of atoms readily available in a compound. For example, the chemical arrangement of peridot would be (Mg,Fe)2SiO4, and for diamonds, it would be C, for carbon. After this has been decided, the gem then has to be classified under the crystal system, so they might be monoclinic, trigonal, or cubic (as in cubic zirconia).

Further Classification

After this is completed, it's a case of putting the gem into it's group, species, and variety. For instance, aquamarine (which is blue), heliodor (which is yellow), and bixbite (which is red) all come under the same variety of stone, known as beryl.

And that's not even the end of it! Further classifications involve going into depth about the gemstones fracture, cleavage, lustre, specific gravity, dispersion, and refractive index, to name but a few. And if the stone itself contains defects or other materials, it is known as having inclusions. So it's really a very complicated task.

Strangely, you would think that due the complexities of the classification system, precious stones were more expensive than semi-precious stones. Although this can be true, it is not in every single case. Garnets are classed as semi-precious, and are quite reasonably priced. However, a certain type of garnet called tsavorite is actually more expensive than an emerald of medium quality.